1. Field of the Invention
The present invention relates in general to regulating power supplies, and in particular to a system and method for load-balancing within a parallel power supply. Still more particularly, the present invention relates to an open-loop system in which an external voltage source provides an independent reference for balancing a current load within a parallel power supply.
2. Description of Related Art
Conventional power supply systems often utilize multiple power supply circuits connected in parallel for improving load variation adaptability and increased reliability. Examples of such parallel power supply systems can be found in Malik, U.S. Pat. No. 5,319,536, Gerner, U.S. Pat. No. 5,266,838, and Line; U.S. Pat. No. 5,745,670.
Referring now to FIGS. 1, 2, and 3, conventional parallel power supply systems 101, 201, and 301 are illustrated. As shown in FIG. 1, parallel power supply system 101 includes two parallel power supply modules, PS1102 and PS2104 that provide independent sources of power to a system load 106. PS1102 and PS2104 generate output currents I1108 and I2110, respectively. Included among the advantages provided by the parallel configuration of power supply system 101 over a unitary power supply system are reduced power supply component costs, enhanced operational flexibility, and improved system reliability.
In the interest of uniformity of device stress and corresponding reduced device failure rates, it is desirable to maintain a balanced power output provided by parallel power sources such as PS1102 and PS2104. FIGS. 2 and 3 illustrate conventional systems for achieving such a balance under varying electrical loads.
As depicted in FIG. 2, power supply system 201 includes parallel power supply modules PS1202 and PS2204 that operate in a "droop" current sharing mode in supplying power to a system load 206. PS1202 and PS2204 maintain a required system voltage level, V.sub.OUT at a common output node 215. Decoupling diodes D1212 and D2214 serve as decoupling barriers between supply-side nodes 217 and 219 and common output node 215. In order to achieve the desired operating balance between PS1202 and PS2204, the current, I1, through node 217 should be approximately equal to the current, I2, flowing through node 219 (one half of the I.sub.T flowing through common output node 215).
As further illustrated in FIG. 2, power supply system 201 includes circuitry within each of the power supply modules 202 and 204 for maintaining a balance in the relative current load borne by each supply. The technique by which such current sharing is achieved in power supply system 201 is commonly referred to in the art as droop current sharing. As depicted in FIG. 2, PS1202 and PS2204 each include a variable resistor (R1220 and R2222 respectively) which serves as a setting and adjustment mechanism for the respective output voltages at nodes 217 and 219. Error amplifiers U1224 and U2226 are configured as negative feedback devices such that as the current drawn by system load 206 from PS1202 and PS2204 increases, the output voltage level at output node 215 decreases.
The current balancing achieved by the closed-loop voltage "droop" effect relies on very precise and symmetric device mirroring between the individual power supply modules PS1202 and PS2204. Such precision is costly and difficult to achieve. Dynamic variations in system load 206 further magnify imbalances between I1 and I2 and result in a wider tolerance required for current imbalance within power supply system 201.
FIG. 3 depicts an alternate closed-loop current sharing technique known in the art as "active current sharing." As illustrated in FIG. 3, power supply system 301 includes a pair of parallel power supply modules PS1302 and PS2304 each having a current sense resistor (R1306 and R2308). Active current sharing at current sharing feedback node 310 is achieved by utilizing the voltage references developed across R1306 and R2308. For example, if the voltage output of PS2304 at supply-side node 314 is higher than that at supply-side node 312 within PS1302, the voltage differential across current sense resistors R1306 and R2308 indicates that PS2304 is providing the majority of the current to system load 306.
The imbalance causes the voltage drop across current sense resistor 308 to be higher than the voltage drop across current sense resistor 306. A higher voltage drop across R2308 results in an increase in the voltage level at current share node 310. This increase in current share node 310 voltage will increase the driving voltages or amplifiers 316 and 317 and will force a higher current to flow through resistor 320 and transistor 318. An increased voltage drop across resistor 320 will result in a higher voltage at node 312. This process continues until both the power supplies PS1302 and PS2304 start current sharing. In this manner, whichever of PS1302 or PS2304 has a higher voltage becomes the master and the other become a slave.
The load balancing within power supply system 301 thus relies on closed-loop feedback provided between PS1302 and PS2304 at current sharing feedback node 310. A varying current through activated transistor 318 or transistor 319 develops an offset voltage across resistors 320 or 321 which causes the output voltage of PS1302 or PS2304 to increase until both PS1302 and PS2304 have equal output voltage levels and thus provide equal current to system load 306.
By providing feedback between each of its constituent power supply modules, the active current sharing system depicted in FIG. 3 provides a more accurately balanced current load between its respective power supply modules than the droop current sharing system depicted in FIG. 2. However, this active sharing method requires expensive overhead components and devices such as very precise and low resistance current sense resistors (R1306 and R2308), and low-offset amplifiers (U1322 and U2324) within each power supply module. This closed-loop feedback approach relies on interdependencies among parallel power supplies to ensure accurate load balancing. Low tolerance devices, such as amplifiers with very low offsets are required to achieve a reliable interdependence.
It would therefore be desirable to provide an system and method for maintaining an accurate load balance for parallel power supplies without relying on interdependent operating parameters among individual power supply modules.